Propylene polymer films are universally used as various packaging material.
However, propylene polymer films of this type have tackiness and tend to undergo blocking. This problem impairs working property in the molding and subsequent processing of the films. In addition, when these films are used, for example, in packaging, troubles such as poor bag openability, etc. tend to occur.
In an attempt to remove these defects, it was proposed, for example, to incorporate additives, such as active silicic acid anhydride (Japanese Patent Publication No. 16908/64), kaolinite (Japanese Patent Publication No. 24523/68), and finely pulverized zeolite powder (Japanese Patent Publication No. 16134/77), in a propylene polymer.
These prior methods exhibit some effect in regard to improve antiblocking property, but cannot fully meet other requirements which have become strict in recent years, as described below.
Firstly, the transparency of films has been regarded as important to secure aesthetic beauty. The antiblocking property of the films can be increased by increasing the amount of the antiblocking agent added. But with increasing amount of the antiblocking agent, its dispersion becomes poor and reduces the transparency of the resulting films.
Secondly, since the present-day film-forming apparatus is adapted to operate at high speeds, the windability of film during film formation is a very important factor. The slipperiness of films during wind-up is poor, and the films cannot be well wound up. Thus, many unacceptable film products result. The film slipperiness has not yet been improved satisfactorily by adding various lubricants.
Thirdly, with regard to the film slipperiness after film formation, for example the bag openability of an article such as a bag formed from the film, etc., an attempt to improve it has previously been made by adding a higher fatty acid amide, etc. as a lubricant so as to bleed out the lubricant onto the film surface. However, if the lubricant is added in a large amount in order to increase the extent of improvement of the slipperiness, whitening occurs to impair the transparency of the film.
On the other hand, catalysts for producing the propylene polymers have been much improved. For example, as a result of increasing productivity by increasing the polymer concentration in a polymerization vessel, and of preventing troubles such as the adhesion of polymer in the polymerization vessel, the poor withdrawal of polymer from the polymerization vessel, etc., it has become possible to obtain polymers in the form of true spherical particles having good powder properties (see, for example, Japanese Laid-Open Patent Application Nos. 34478/72, 40293/79, 180612/82, and 811/81).
Polymer powders in the form of true spherical particles obtained as a result of such improvements in the polymerization process have smooth surfaces. Hence, when such polymer powders are mixed with a conventional inorganic antiblocking agent, the antiblocking agent is dispersed badly in the polymer powders, and the transparency of the resulting film is reduced. Furthermore, many fish eyes occur in the film to reduce the appearance of the film markedly. In an attempt to improve the dispersion of the antiblocking agent such as silicon dioxide, it was proposed, for example, to use a magnesium or aluminum salt of a fatty acid in combination with the antiblocking agent (Japanese Laid-Open Patent Application No. 225142/83), or to use an inorganic fine powder having a layered structure in combination with the antiblocking agent (Japanese Laid-Open Patent Application No. 18747/82). According to these methods, however, the appearance of the film has not been improved satisfactorily.